Fischer-Tropsch products can be upgraded before being sold as products. The usual processes that are used are hydrocracking to make distillate fuels such as diesel and jet fuel, naphtha, and feeds for lube processing. These products can also be upgraded by wax isomerization to make lube base oils. Finally, the light naphtha can be reformed to make aromatics for use in gasoline or petrochemicals.
Several catalysts used in these upgrading processes require low levels of heteroatoms for efficient operation. The key heteroatoms that must be controlled to low levels are nitrogen and oxygen. Nitrogen is the most serious catalyst poison, while oxygen is a lesser concern. The presence of nitrogen-containing impurities in a feedstock causes the reactions to be performed at higher than desired hydroprocessing reactor temperatures, with a serious reduction in the yield of valuable products.
Products from Fischer-Tropsch synthesis are well known to contain oxygen impurities and have little or no sulfur. It was unknown that Fischer-Tropsch products can contain nitrogen. If oxygen is the only impurity to be considered, the upgrading can be quite mild, as described in EP 583 836 B1 and EP 668 342 A1. These patents speak of “mild hydrogenation, under conditions such that substantially no isomerization or hydrocracking of hydrocarbons occurs” (lines 50-55 in 583 836 B1). These mild conditions typically remove sulfur and oxygen impurities, but not nitrogen impurities. Likewise, U.S. Pat. No. 4,943,672 describes severe hydrotreating to improve the processing of Fischer-Tropsch wax, but states that Fischer-Tropsch wax contains essentially no nitrogen or sulfur.